One of the first people Shep Gordon met as a 22-year-old arriving in LA was Janis Joplin. She belted him across the face. And believe it or not, his life only got more interesting.

After graduating from the University at Buffalo (SUNY), Gordon drove to Los Angeles and moved into the first hotel he saw. It turned out to be the Hollywood Landmark which, unbeknownst to Gordon, was famous for its rock ’n’ roll residents.

On his first night, the newbie was awoken by a commotion. He headed downstairs to find two people loudly having sex poolside. The woman slapped him for interrupting. It was Joplin.

Gordon befriended her, as well as another famous resident, Jimi Hendrix, who asked Gordon what he did for a living. He didn’t have an answer. After discovering Gordon was Jewish — like Bob Dylan’s manager, Albert Grossman, and the Beatles’ Brian Epstein — Hendrix suggested he become a manager.

Among the first acts Gordon signed was Alice Cooper. Today, some 45 years later, Gordon still handles the shock-rocker and has become one of the most influential figures in the entertainment business — while remaining anonymous to the general public.

That changes Friday with the documentary “Supermensch: The Legend of Shep Gordon.” It’s directed by Mike Myers, who met Gordon, now 68, while arranging an Alice Cooper appearance in “Wayne’s World.”

“My management advice to Mike was, ‘Are you out of your mind? Why would you stop your career to do a documentary about me, who nobody cares about?’ ” Gordon tells The Post.

Watch the film and you will care. In it, Sylvester Stallone, Michael Douglas and others recount Gordon’s life taking care of artists including Blondie and Luther Vandross.

He has more stories than the film can hold. There was the time he and Cary Grant shared joint custody of a cat. And the time he was vacationing on a Fiji island inhabited by one other couple. Gordon’s computer wasn’t working, so he called the front desk. A few minutes later, one of the two other visitors on the island showed up to help — Steve Jobs.

Then there were Gordon’s genius promotional schemes. In the 1970s, the manager had difficulty breaking Canadian songbird Anne Murray because she was seen as white-bread. So Gordon arranged for John Lennon, Cooper and Harry Nilsson to attend one of Murray’s LA gigs to lend a sense of cool.

Gordon says his favorite scheme involved an Alice Cooper show in London early in the rocker’s career. Cooper was booked to play Wembley Arena. A few days before the show, Gordon was horrified to learn only 57 tickets had been sold.

The manager rented a flatbed truck and, on the back, placed a billboard depicting Cooper, nude, with a snake crawling over him. Gordon ordered the driver to head to London’s busiest intersection and pretend to break down, stopping traffic. The stunt snarled roads and landed the driver in jail. The government considered banning the rocker. In two days, the concert sold out.

At another show, Gordon wanted the audience to give Cooper a standing ovation to improve the reviews.

“We threw money into the first row,” he says. “When all those people jumped up, so did everyone else.”

Gordon is also credited with creating the celebrity-chef phenomenon. The foodie met chef Roger Vergé and was appalled to learn what little respect top culinary talents got. So he turned them into rock stars.

Working in the early 1990s with Emeril Lagasse, Alice Waters and others, Gordon landed his clients TV appearances. Worldwide fame soon followed.

“That’s the thing that I feel proudest of,” Gordon says. “I was passing on making $500,000 a night [with bands],to work with a chef who made $50,000 a year.”

Today, he’s partially retired, lives in Hawaii and owns a few restaurants.

“The only thing I have to gain from this movie is fame, which I don’t want,” he says.

And that’s one of Gordon’s charms. He dreams big for his clients, small for himself.